The invention relates to intensity modulation of optical signals, more especially but not exclusively to wavelength converters and demultiplexers of optical signals carrying data encoded by intensity modulation.
A typical wavelength converter transfers an intensity modulation in a first signal of a first wavelength to a second signal of a second wavelength. This is useful for converting return zero (RZ) or non-return-zero (NRZ) data from one wavelength to another wavelength, as is a generally useful in a wavelength division multiplexed (WDM) environment, for example for an optical cross connect (OXC).
It is known that a wavelength converter can be based on cross phase modulation (XPM) or cross gain modulation (XGM) in a nonlinear medium, such as a semiconductor optical amplifier (SOA) or nonlinear optical fiber.
FIG. 1 of the accompanying drawings is a generalized schematic depiction of a two-stage wavelength converter employing either XPM or XGM, as exemplified by a number of prior art devices.
The generic prior art wavelength converter comprises a first stage 10 connected to an input 12 for receiving a first optical signal of a first frequency f1 (or wavelength λ1) and to a further input 14 for receiving a second optical signal of a second frequency f2 (or wavelength λ2). The first stage 10 comprises some kind of nonlinear medium for inducing XPM or XGM of the first signal by the second signal, thereby transferring any intensity modulation contained in the second signal to the first signal. Typically the first optical signal is input as a continuous wave (CW) signal, that may be termed a carrier signal, and the second optical signal is input as a data carrying intensity modulated (IM) signal, that may be termed a drive signal.
Various different components have been used in the prior art to provide XPM in the first stage. Examples are a single semiconductor optical amplifier (SOA) for XPM [1–3] or XGM [7–10, 22], a Mach-Zehnder interferometer (MZI) with SOAs in each arm for XPM [11, 23], and dispersion shifted fiber (DSF) for XPM [4–6].
The device further comprises a second stage 20 having an input connected by an optical path 16 to receive the second optical signal after its XPM from the first stage. The purpose of the second stage is to provide a filter that spectrally enhances the output of the first stage to improve signal quality of the data carried by the second optical signal, as measured by some parameter such as bit error rate (BER). (It is noted that a simple wavelength converter will have no second stage and thus consist only of the above described first stage.) The second stage exploits the fact that the XPM impresses the IM of the second optical signal onto the first optical signal not only as an IM, but also as a phase modulation (PM), i.e. a chirp. The chirp is related to the nonlinear phase shift induced by the XPM. The second stage is designed to generate a further IM contribution responsive to the chirp in the first optical signal, and to generate a further IM which sums with the IM already impressed on the first optical signal in the first stage prior to outputting the signal to an output 18.
To increase the effective frequency response of the first stage, the second stage is designed with a linear transfer function. The linear transfer function of the second stage transforms the PM into IM in a manner that weights the IM components from the first stage output to flatten the frequency response of the overall device.
Various different components have been used in the prior art for the second stage. Examples are birefringent fiber for XPM [1], a fiber Bragg grating (FBGs) for XPM [2, 3, 11, 24] or XGM [7–10], and a loop mirror filter for XPM [4–6].
In summary, a variety of two stage devices have been proposed for wavelength conversion that are based on XPM and XGM. Several of these devices use the PM generated in a wavelength converting first stage to drive a second stage, the second stage having a linear transfer function that compensates for the inherently slow frequency response in the first stage.